Research Papers. 1. B.R. Manjunath, P. Sadasivamurthy, P.V. Reddy, Karickal R. Haridas, Studies on improving performance of PVC

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1 33 Research Papers 1. B.R. Manjunath, P. Sadasivamurthy, P.V. Reddy, Karickal R. Haridas, Studies on improving performance of PVC compositions for electrical applications, The Chemist, cable sheathing [Accepted (in press)]. 2. B.R. Manjunath, P. Sadasivamurthy, P.V. Reddy, Karickal R. Haridas, Studies on cenospheres as fillers for PVC compounds for applications in electrical cables, The Chemist, [Communicated]. 3. Presented paper Cenospheres as possible fillers for PVC compounds in electrical cable industry at 24th Kerala Science Congress, Kottayam, Kerala, 2012; the poster was conferred the Best Poster Award in Chemical Sciences section. 4. Presented paper Investigation of cenospheres as possible fillers for PVC compounds in electrical cable industry, at National Seminar on Social Relevance of Chemical Sciences, Kuvempu University, Shimoga, Karnataka, 2011.

2 34 I am inclined to think that the development of polymerization is, perhaps, the biggest thing Chemistry has done, where it has had the biggest effect on everyday life. The world would be a totally different place without artificial fibers, plastics, elastomers, etc. Even in the field of electronics, what would you do without insulation? And there, you come back to polymers again!. Lord Todd, President of the Royal Society of London, in answer to the question, What do you think has been Chemistry s biggest contribution to Science, to Society? [Quoted in Chemical Engg. News, 58 (40) Pp. 29, 1980].

3 35 Chapter 1 INTRODUCTION

4 Introduction Types of flame-retardants Mechanism of action of flame-retardants Performance criteria for and choice of flame-retardants Production and uses of flame-retardants and flameretarded polymers Plasticizers Formation of toxic products on heating or combustion of flame-retarded products Overview of exposure and hazards to humans and the environment Regulations with respect to flame-retardants Recommendations for the protection of human health and the environment Further research

5 Introduction Accidental fire is an ever-present hazard. In present-day living, there is a rapidly increasing development in the size and number of buildings, skyscrapers, warehouses and methods of transport. Carpeting, furnishings, equipment, increased presence of electrical cables, oil and gas for heating and so on, all increase the fire load in a building. New technologies, new processes and new applications introduce new fire hazards (e.g., new ignition sources such as welding sparks and short circuits) [1]. Though modern firefighting techniques, equipment and building design reduce the destruction due to fires, a high fuel load in either a residential or a commercial building can offset even the best of building constructions [2(i)]. There is an everexisting need to improve upon the flame-retardance of the basic material, the polymer, used in various items, ranging from electrical cables to wall coverings to clothes to furniture in buildings. Each year, over three million fires leading to over 1,00,000 injuries and 15,000 deaths are reported worldwide. The direct property losses exceed $8 billion and the total annual cost has been estimated at over $100 billion. Personal losses occur mostly in residences where furniture, wall coverings and clothes are frequently the fuel. Large financial losses occur in commercial structures such as office buildings and warehouses. Fires also occur in aeroplanes, buses and trains [2(ii)]. In order to provide additional protection from fires and to increase escape time when a fire occurs, methods to enhance the flame-retardance of consumer goods have been developed. Flame-retardants are chemicals added to polymeric materials, both natural and synthetic, to enhance flameretardance properties. They may be physically blended with or chemically

6 38 bonded to the host polymer. Generally, they either lower ignition susceptibility or make the flame spread slower, once ignition has occurred. Flame-retardant systems for synthetic or organic polymers act in five basic ways: (i) gas dilution; (ii) thermal quenching; (iii) protective coating; (iv) physical dilution; and (v) chemical interaction [3]; or through a combination of these mechanisms. 1. Inert gas dilution involves using additives that produce large volumes of non-combustible gases on decomposition. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Metal hydroxides, metal salts and some nitrogen compounds function in this way. 2. Thermal quenching is the result of endothermic decomposition of the flame-retardant. Metal hydroxides, metal salts and nitrogen compounds act to decrease surface temperature and the rate of burning. 3. Some flame-retardants form a protective liquid or char barrier. This limits the amount of polymer available to the flame front and/or acts as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphorus compounds and intumescent systems based on melamine and other nitrogen compounds are examples of this category. 4. Inert fillers (glass fibers and microspheres) and minerals (talc) act as thermal sinks to increase the heat capacity of the polymer or reduce its fuel content. 5. Halogens and some phosphorus flame-retardants act by chemical interaction. The flame-retardant dissociates into radical species that compete with chain-propagating steps in the combustion process.

7 39 Chemicals that are used as flame-retardants can be inorganic, organic, mineral, halogen-containing or phosphorus-containing. The term flameretardant represents a class of use and not a class of chemical structure [3]. Preventive flame protection, including the use of flame-retardants, has been practiced since ancient times. Some examples of early historical developments in flame-retardants are shown in Table 1.1 [4]. Table 1.1. Early historical fire-retardant developments Development Date Alum used to reduce the flammability of wood by the Egyptians The Romans used a mixture of alum and vinegar on wood Mixture of clay and gypsum used to reduce flammability of theatre curtains Mixture of alum, ferrous sulfate and borax used on wood and textiles by Wyld in Britain Alum used to reduce flammability of balloons Gay-Lussac reported a mixture of (NH4)3PO4, NH4Cl and borax to be effective on linen and hemp Perkin described a flame-retardant treatment for cotton using a mixture of sodium stannate and ammonium sulfate About 450 BC About 200 BC The advent of synthetic polymers in the last century was of special significance, since the water-soluble inorganic salts in use up to that time were of little or no utility in these largely hydrophobic materials. This led to modern research concentrating on the development of polymer-compatible flame-retardants. With the increasing use of thermoplastics and thermosets on a large scale for applications in electrical engineering and electronics, building, transportation, new flame-retardant systems came to be developed. They

8 40 mainly consisted of inorganic and organic compounds based on bromine, chlorine, phosphorus, nitrogen, boron, and metallic oxides and hydroxides. Today, there are flame-retardant systems developed to fulfill the multiple flammability requirements of the above-mentioned applications Types of flame-retardants A distinction is made between reactive and additive flame-retardants. Reactive flame-retardants are reactive components, chemically built into a polymer molecule. polymer either Additive flame-retardants are incorporated into the prior to, during or (most frequently) following polymerization. There are three main families of flame-retardant chemicals [1,2(ii)(iv),5,6]. 1. The main inorganic flame-retardants are aluminum trihydroxide (ATH), antimony trioxide, magnesium hydroxide, ammonium polyphosphate and red phosphorus. This group represents about 50% by volume of the worldwide flame-retardant production. Some of these chemicals are also used as flame-retardant synergists, of which antimony trioxide is the most important [7]. 2 Halogenated products are based primarily on chlorine and bromine. This group represents about 25% by volume of the worldwide production [8]. 3. Organophosphorus products are primarily phosphate esters and represent about 20% by volume of the worldwide production. Products containing phosphorus, chlorine and/or bromine are also important. In addition, nitrogen-based flame-retardants are used for a limited number of polymers.

9 Inorganic flame-retardants Very few inorganic compounds are suitable for use as flameretardants in plastics. This is because they are usually too inert to be effective in the range of decomposition temperatures of plastics (between 150 and 400 oc). One major disadvantage of inorganic flame-retardants is hygroscopicity- this is sometimes sought to be overcome by adding fillers such as clay which reduce water absorption. Metal hydroxides form the largest class of all flame-retardants used commercially today and is employed alone or in combination with other flame-retardants to achieve necessary improvements in flame-retardancy. Antimony compounds are used as synergistic co-additives in combination with halogen compounds, facilitating a cut in total flame-retardant levels needed to achieve a desired level of flame-retardancy. To a limited extent, compounds of other metals also act as synergists with halogen compounds. They may be used alone but are most commonly used with antimony trioxide to enhance other characteristics, for example, smoke reduction or afterglow suppression. Ionic compounds have a very long history as flame-retardants for wool- or cellulose-based products. Inorganic phosphorus compounds are primarily used in polyamides and phenolic resins, or as components in intumescent formulations Metal hydroxides Metal hydroxides function in both the condensed and gas phases of a fire by absorbing heat and decomposing to release their water of hydration. This process cools both the polymer and the flame and dilutes the flammable gas mixture. The very high concentrations (50 to 80%) required to impart flame-retardancy often adversely affect the mechanical properties of the polymer into which they are incorporated.

10 42 ATH is the largest volume flame-retardant in use today. It decomposes when exposed to temperatures over 200 C, which limits the polymers into which it can be incorporated. Magnesium hydroxide is stable to temperatures above 300 C and can be processed into several polymers Antimony compounds Antimony trioxide is not a flame-retardant per se, but is used as a synergist. It is utilized in plastics, rubbers, textiles, paper and paints, typically 2-10% by weight, with organochlorine and organobromine compounds to diminish the flammability of a wide range of plastics and textiles [9]. Antimony oxides and antimonates must be converted to volatile species. This is usually accomplished by release of halogen acids at fire temperatures. The halogen acids react with the antimony-containing materials to form antimony trihalide and/or antimony halide/oxide. These materials act both in the substrate (condensed phase) and in the flame to suppress flame propagation. In the condensed phase, they promote char formation, which acts as a physical barrier to flame and inhibits the volatilization of flammable materials. In the flame, the antimony halides and halide oxides, generated in sufficient volume, provide an inert gas blanket over the substrate, thus excluding oxygen and preventing flame spread. These compounds alter the chemical reactions occurring at fire temperatures in the flame, thus reducing the ease with which oxygen can combine with the volatile products. It is also suggested that antimony oxychloride or trichloride reduces the rate at which the halogen leaves the flame zone, thus increasing the probability of reaction with the reactive species. Antimony trichloride probably evolves heavy vapors which form a layer over the condensed phase, stop oxygen attack and thus choke the flame. It is also

11 43 assumed that the liquid and solid antimony trichloride particles contained in the gas phase reduce the energy content of the flames by wall or surface effects [10]. Other antimony compounds include antimony pentoxide, available primarily as a stable colloid or as a redispersible powder. It is designed primarily for highly specialized applications, although manufacturers suggest it has potential use in fiber and fabric treatment. Sodium antimonate is recommended for formulations in which deep tone colors are required or where antimony trioxide may promote unwanted chemical reactions Boron compounds Within the class of boron compounds, by far the most widely used is boric acid. Boric acid (H3BO3) and sodium borate (borax) (Na2B4O7.10H2O) are the two flame-retardants with the longest history, and are used primarily with cellulosic material, e.g., cotton and paper. Both products are effective, but their use is limited to products for which non-durable flame-retardancy is acceptable since both are very water-soluble. Zinc borate, however, is water-insoluble and is mostly used in plastics and rubber products. It is used either as a complete or partial replacement for antimony oxide in PVC, nylon, polyolefin, epoxy, EPDM, etc. In most systems, it displays synergism with antimony oxide. Zinc borate can function as a flame-retardant, smoke-suppressant and anti-arcing agent in condensed phase. Recently, zinc borate has also been used in halogen-free, fire-retardant polymers Other metal compounds Molybdenum compounds have been used as flame-retardants in cellulosic materials for many years and more recently with other polymers, mainly as smoke-suppressants [1]. They appear to function as condensed-

12 44 phase flame-retardants [11]. Titanium and zirconium compounds are used for textiles, especially wool [12]. Zinc compounds, such as zinc stannate and zinc hydroxy-stannate, are also used as synergists and as partial replacements for antimony trioxide Phosphorus compounds Red phosphorus and ammonium polyphosphate (APP) are used in various plastics. Red phosphorus was first introduced in polyurethane foams and found to be very effective as a flame-retardant. It is now used particularly for polyamides and phenolic applications. The flame-retarding effect is due, in all probability, to the oxidation of elemental phosphorus during the combustion process to phosphoric acid or phosphorus pentoxide. The latter acts by the formation of a carbonaceous layer in the condensed phase. The formation of fragments that act by interrupting the radical chain mechanism is also likely. Ammonium polyphosphate is mainly applied in intumescent coatings and paints. Intumescent systems puff up to produce foams. Because of this characteristic, they are used to protect materials such as wood and plastics that are combustible and those like steel that lose their strength when exposed to high temperatures. Intumescent agents have been available commercially for many years and are used mainly as fire-protective coatings. They are now used as flame-retardant systems for plastics by incorporating the intumescent components in the polymer matrix, mainly polyolefins, particularly polypropylene [1] Other inorganic flame-retardants Other inorganic flame-retardants, including ammonium sulfamate (NH4SONH2) and ammonium bromide (NH4Br), are used primarily with cellulose-based products and in forest fire-fighting [5].

13 Halogenated organic flame-retardants Halogenated flame-retardants can be divided into three classes: aromatic, aliphatic and cycloaliphatic. Bromine and chlorine compounds are the only halogen compounds having commercial significance as flameretardant chemicals. Fluorine compounds are expensive and, except in special cases, are ineffective because the C-F bond is too strong. Iodine compounds, although effective, are expensive and too unstable to be useful [2(ii), 13]. The brominated flame-retardants are much more numerous than the chlorinated types because of their higher efficacy [14]. With respect to processability, halogenated flame-retardants vary in their thermal stability. In general, brominated aromatic flame-retardants are thermally more stable than chlorinated aliphatics, which, in turn, are thermally more stable than brominated aliphatics. Brominated aromatic compounds can be used in thermoplastics at fairly high temperatures without the use of stabilizers and at very high temperatures with stabilizers. The thermal stability of the chlorinated and brominated aliphatics is such that, with few exceptions, they must be used with thermal stabilizers, such as a tin compound. Halogenated flame-retardants are either added to or reacted with the base polymer. Additive flame-retardants are those that do not react in the application designated. There are a few compounds that can be used as an additive in one application and as a reactive in another; tetrabromobisphenol A is the most notable example. Reactive flame-retardants become a part of the polymer either by becoming a part of the backbone or by grafting onto the backbone. The choice of a reactive flame-retardant is more complex than the choice of an additive type. The development of systems based on reactive flame-retardants is more expensive for the manufacturer, who in effect has to develop novel co-polymers with the desired chemical, physical

14 46 and mechanical properties, as well as the appropriate degree of flameretardance [2(i),13]. Synergists such as antimony oxides are frequently used with halogenated flame-retardants Brominated flame-retardants Bromine-based flame-retardants are highly brominated organic compounds with a relative molecular mass ranging from 200 to that of large molecule polymers. They usually contain 50 to 85% (by weight) of bromine [14]. The highest volume brominated flame-retardant in use today is tetrabromobisphenol-a (TBBPA) [15], followed by decabromodiphenyl ether (DeBDE) [16]. Both of these flame-retardants are aromatic compounds. The primary use of TBBPA is as a reactive intermediate in the production of flame-retarded epoxy resins, used in printed circuit boards [15]. A secondary use for TBBPA is as an additive flame-retardant in ABS systems. DeBDE is the second largest volume brominated flame-retardant and is the largest volume brominated flame-retardant used solely as an additive. The greatest use (by volume) of DeBDE is in high-impact polystyrene, which is primarily used to produce television cabinets. Secondary uses include ABS, engineering thermoplastics, polyolefins, thermosets, PVC and elastomers. DeBDE is also widely used in textile applications as the flame-retardant in latex-based back coatings [2(ii)]. Hexabromocyclododecane (HBCD), a major brominated cycloaliphatic flame-retardant, is primarily used in polystyrene foam. It is also used to flame-retard textiles Chlorinated flame-retardants Chlorine-containing flame-retardants belong to three chemical groups: aliphatic, cycloaliphatic and aromatic compounds. Chlorinated

15 47 paraffins are by far the most widely used aliphatic chlorine-containing flameretardants. They have applications in plastics, fabrics, paints and coatings [17]. Bis(hexachlorocyclopentadieno)cyclo-octane is a flame-retardant having unusually good thermal stability for a chlorinated cycloaliphatic. In fact, this compound is comparable in thermal stability to brominated aromatics in some applications. It is used in several polymers, especially polyamides and polyolefins for wire and cable applications. Its principal drawback is the relatively high use levels required, compared to some brominated flame-retardants [2(ii)] Organophosphorus flame-retardants One of the principal classes of flame-retardants used in plastics and textiles is that of phosphorus, phosphorus-nitrogen and phosphorus-halogen compounds. Phosphate esters, with or without halogen, are the predominant phosphorus-based flame-retardants in use. For textiles, phosphorus-containing materials are by far the most important class of compounds used to impart durable flame-resistance to cellulose. These textiles flame-retardant finishes usually also contain nitrogen or halogen, or sometimes both [5,12] Non-halogenated compounds Although many phosphorus derivatives have flame-retardant properties, the number of those with commercial importance is limited. Some are additive and some, reactive. The major groups of additive organophosphorus compounds are phosphate esters, polyols, phosphonium derivatives and phosphonates. The phosphate esters include trialkyl derivatives such as triethyl or trioctyl phosphate, triaryl derivatives such as

16 48 triphenyl phosphate and aryl-alkyl derivatives such as 2-ethylhexyl-diphenyl phosphate. The flame-retardancy of cellulosic products can be improved through the application of phosphonium salts. The flame-retardant treatments attained by phosphorylation of cellulose in the presence of a nitrogen compound are also of importance [12]. Plasticizers are mixed into polymers to increase flexibility and workability. The esters formed by reaction of the three functional groups of phosphoric acid with alcohols or phenols are excellent plasticizers. The phosphoric acid esters are also remarkable flame-retardants, and for this reason are extensively used in plastics [17]. Aryl phosphate plasticizers are used in PVC-based products. They are also used as lubricants for industrial air compressors and gas turbines. Miscellaneous uses of aryl phosphates are as pigment dispersants and peroxide carriers, and as additives in adhesives, lacquer coatings and wood preservatives [18] Halogenated phosphates In addition to the above types, flame-retardants containing both chlorine and phosphorus or bromine and phosphorus are used widely. Halogenated phosphorus flame-retardants combine the flameretardant properties of both the halogen and the phosphorus groups. In addition, the halogens reduce the vapor pressure and water solubility of the flame-retardant, thereby contributing to the retention of the flame-retardant in the polymer. One of the largest selling members of this group, tris(1-chloro-2propyl) phosphate (TCPP) is used in polyurethane foam. Tris(2-chloroethyl) phosphate is used in the manufacture of polyester resins, polyacrylates,

17 49 polyurethanes and cellulose derivatives. The most widely used bromine- and phosphorus-containing flame-retardant used to be tris(2,3- dibromopropyl)phosphate, but it was withdrawn from use in many countries due to carcinogenic properties in animals [2(iii),18] Nitrogen-based flame-retardants Nitrogen-based compounds can be employed in flame-retardant systems or form part of intumescent flame-retardant formulations. Nitrogenbased flame-retardants are used primarily in nitrogen-containing polymers such as polyurethanes and polyamides. They are also utilized in PVC and polyolefins and in the formulation of intumescent paint systems [19]. Melamine, melamine cyanurate, other melamine salts and guanidine compounds are currently the most used group of nitrogen-containing flameretardants. Melamine is used as a flame-retardant additive for polypropylene and polyethylene. Melamine cyanurate is employed commercially as a flame-retardant for polyamides and terephthalates (PET/PBT) and is being developed for use in epoxy and polyurethane resins. Melamine phosphate is also used as a flame-retardant for terephthalates (PET/PBT) and is currently being developed for use in epoxy and polyurethane flame-retardant formulations. Also in the development stages for use as flame-retardant additives are melamine salts and melamine formaldehyde for their application in thermoset resins [20] Requirements of an ideal flame-retardant Following are some of the requirements of an ideal flame-retardant: 1. It should be compatible with the base polymer. 2. It should be easy to incorporate. 3. It should not alter the mechanical properties of the polymer.

18 50 4. It should not bloom or bleach and possess good resistance to aging. 5. It must be stable at processing and service temperatures. 6. It should be effective in small quantities and must be noncorrosive. 7. It must be odor-free and free from harmful effects on human physiology and environment, and 8. It must emit low smoke and must be cost-effective Mechanism of action of flame-retardants The mechanism of action of flame-retardants and smoke suppressants is indeed quite complex. However, a general outline of the same is given in the ensuing paragraphs General aspects To understand flame-retardants, it is necessary to first understand fire. Fire is a gas-phase reaction. Thus, in order for a substance to burn, it must become a gas. In the case of a candle, the wax melts and migrates up the wick by capillary action. The wax is pyrolysed to volatile hydrocarbon fragments on the wick's surface at C. There is no oxygen at the nucleus of the flame. Some of the hydrocarbon fragments aromatize to soot particles and, in the luminescent region of the flame, react with water and carbon dioxide to form carbon monoxide. Most of the pyrolysis gases are carried to the exterior of the flame and encounter oxygen diffusing inwards. They react exothermically to produce heat, which melts and decomposes more wax, maintaining the combustion reaction. If there is adequate oxygen, the combustion products from the candle are carbon dioxide and water [21]. Natural and synthetic polymers can ignite on exposure to heat. Ignition occurs either spontaneously or results from an external source such

19 51 as a spark or flame. If the heat evolved by the flame is sufficient to keep the decomposition rate of the polymer above that required to maintain the evolved combustibles within the flammability limits, then a self-sustaining combustion cycle will be established, figure 1.1. Figure 1.1. The combustion process This self-sustaining combustion cycle occurs across both the gas and condensed phases. Fire-retardants act to break this cycle by affecting chemical and/or physical processes occurring in one or both of the phases. There are a number of ways in which the self-sustaining combustion cycle can be interrupted. Whatever the method used, the end effect is to reduce the rate of heat transfer to the polymer and thus remove the fuel supply. Troitzsch [1] describes the general physical and chemical mechanisms of flame-retardant action, in both the gas and condensed phases and the behavior of flame-retardants. Fundamentally, four processes are involved in polymer flammability: preheating, decomposition, ignition and combustion/propagation. Preheating involves heating of the material by means of an external source, which raises the temperature of the material at a rate dependent upon the thermal intensity of the ignition source, the thermal conductivity of the material, the specific heat of the material, and the latent heat of fusion and vaporization of the material. When sufficiently heated, the

20 52 material begins to degrade, i.e., it loses its original properties as the weakest bonds begin to break. Gaseous combustion products are formed, the rate being dependent upon such factors as intensity of external heat, temperature required for decomposition, and rate of decomposition. The concentration of flammable gases increases until it reaches a level that allows sustained oxidation in the presence of the ignition source. The ignition characteristics of the gas and the availability of oxygen are two important variables in any ignition process. After ignition and removal of the ignition source, combustion becomes self-propagating if sufficient heat is generated and is radiated back to the material to continue the decomposition process. The combustion process is governed by such variables as rate of heat generation, rate of heat transfer to the surface, surface area, and rates of decomposition. Flame-retardancy, therefore, can be achieved by eliminating (or improved by retarding) any one of these variables. A flame-retardant should inhibit or even suppress the combustion process. Depending on their nature, flame-retardants can act chemically and/or physically in the solid, liquid or gas phase. They interfere with combustion during a particular stage of this process, i.e. during heating, decomposition, ignition or flame spread [1] Physical action There are several ways in which the combustion process can be retarded by physical action [1]. (a) By cooling. Endothermic processes triggered by additives cool the substrate to a temperature below that required to sustain the combustion process. (b) By formation of a protective layer (coating). The condensed combustible layer can be shielded from the gaseous phase with a

21 53 solid or gaseous protective layer. The condensed phase is thus cooled, smaller quantities of pyrolysis gases are evolved, the oxygen necessary for the combustion process is excluded and heat transfer is impeded. (c) By dilution. The incorporation of inert substances (e.g., fillers) and additives that evolve inert gases on decomposition and dilute the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not exceeded Chemical action The most significant chemical reactions interfering with the combustion process take place in the solid and gas phases [1]. Usually, reactions occur in two phases: (a) Reaction in the gas phase. The free radical mechanism of the combustion process which takes place in the gas phase is interrupted by the flame-retardant. The exothermic processes are thus stopped, the system cools down, and the supply of flammable gases is reduced and eventually completely suppressed. (b) Reaction in the solid phase. Here two types of reaction can take place. Firstly, breakdown of the polymer can be accelerated by the flame-retardant, causing pronounced flow of the polymer and, hence, its withdrawal from the sphere of influence of the flame, which breaks away. Secondly, the flame-retardant can cause a layer of carbon to form on the polymer surface. This can occur, for example, through the dehydrating action of the flame-retardant generating double bonds in the polymer. These form the carbonaceous layer by cyclizing and cross-linking.

22 54 Flame-retardancy is improved by flame-retardants that cause the formation of a surface film of low thermal conductivity and/or high reflectivity, which reduces the rate of heating. It is also improved by flameretardants that might serve as a heat sink by being preferentially decomposed at low temperature. Finally, it is improved by flame-retardant coatings that, upon exposure to heat, intumesce into a foamed surface layer with low thermal conductivity properties. A flame-retardant can promote transformation of a plastic into char and thus limit production of combustible carbon-containing gases. Simultaneously, the char will decrease thermal conductivity of the surface. Flame-retardants can also chemically alter the decomposition products, resulting in a lower concentration of combustible gases. Reduced fuel will result in less heat generation by the flame and may lead to self-extinction. Structural modification of the plastic, or use of an additive flameretardant, might induce decomposition or melting upon exposure to a heat source so that the material shrinks or drips away from the heat source. It is also possible to significantly retard the decomposition process through selection of chemically stable structural components or structural modifications of a polymer. In general, anything that will prevent the formation of a combustible mixture of gases will prevent ignition. However, one may also distinguish those cases in which the flame-retardant or the modified polymer unit, upon exposure to a heat source, will form gas mixtures that will react chemically in the gas phase to inhibit ignition. The goal of flame-retardance in the combustion and propagation stages is to decrease the rate of heat generated or radiated back to the substrate. Any or all of the above-mentioned mechanisms could function to prevent a selfsustaining flame [22].

23 55 Flame-retardancy occurs both as already stated in the vapor phase (by interfering with oxidation through removal of free radicals) and in the condensed phase (charring or altering thermal degradation processes). Phosphorus acts primarily in the condensed phase by promoting charring, presumably through the formation of phosphoric acid and a decreased release of flammable volatiles. However, some reports indicate that certain organic phosphorus compounds may also work in the gas phase by scavenging free radicals. Antimony (which functions only in the presence of a halogen) is believed to work similarly to phosphorus in the condensed phase and combine with halogens in the gas phase to scavenge free radicals that are necessary for combustion. The role of nitrogen is not completely understood. Nitrogen is known to impart flame-retardancy in combination with phosphorus and also by itself, as in polyamides and aminoplasts. Bromine and chlorine act in the gas phase by reacting with free radicals [23]. The mechanism for imparting durable flame-retardance to cellulose is that of increasing the quantity of carbon, or char, formed instead of volatile products of combustion, and flammable tars. Salts that dissociate to form acids or bases upon heating are usually effective flame-retardants. Salts of strong acids and weak bases are the most effective compounds. Ammonium and amine salts are generally effective, as are Lewis acids and bases, either by themselves or when formed in combustion Condensed phase mechanisms The role of phosphorus compounds has been extensively studied. In both cellulose and thermoplastics, phosphorus salts of volatile metals and most organophosphorus compounds are known to be effective flameretardants. The formation of char appears to be the key. For example, although triphenyl phosphate, triphenyl phosphite and triphenyl phosphine

24 56 are all equivalent on a phosphorus basis, the more effective flame-retardant compounds act by forming phosphoric acid, which changes the course of the decomposition of cellulose to form carbon and water [24]. The flame-retardant action of phosphorus compounds in cellulose is believed to proceed by way of initial phosphorylation of the cellulose, probably by initially formed phosphoric or polyphosphoric acid. The phosphorylated cellulose then breaks down to water, phosphoric acid and an unsaturated cellulose analogue, and eventually to char by repetition of these steps. Certain nitrogen compounds such as melamines, guanidines, ureas and other amides appear to catalyze the steps forming cellulose phosphate and are found to enhance or synergize the flame-retardant action of phosphorus on cellulose. In polyethylene terephthalate and polymethyl methacrylate, the mechanism of action of phosphorus-based flame-retardants has been shown to involve both a similar decrease in the amount of combustible volatiles and a similar increase in the amount of residues (aromatic residues and char). The char formed also acts as a physical barrier to heat and gases. In rigid polyurethane foams the action of phosphorus flame-retardants also appears to involve char enhancement. In flexible foam, the mechanism is less wellunderstood [25] Gas-phase mechanisms In addition to the condensed-phase mechanism, phosphorus flame- retardants can exert gas-phase flame-retardant action. It has been demonstrated that trimethyl phosphate retards the velocity of a methaneoxygen flame with about the same molar efficiency as antimony trioxide [2(ii)]. The mechanisms of action can differ depending on the type of compound used as a flame-retardant. The mechanism affects the generation

25 57 of products of combustion, some of which are potentially corrosive and toxic. One of the methods for improving the flame-retardancy of thermoplastic materials is to lower their melting point. This results in the formation of free radical inhibitors in the flame front and causes the material to recede from the flame without burning. Free radical inhibition involves the reduction of gaseous fuels generated by burning materials. Heating of combustible materials results in the generation of hydrogen, oxygen, and hydroxide and peroxide radicals that are subsequently oxidized with flame. Certain flame-retardants act to trap these radicals and thereby prevent their oxidation. Bromine is more effective than chlorine. If the resulting compound is less readily oxidized than the radical that is removed, the result is reduced flammability. Measurements of the limiting oxygen index of polymers show that, in contrast to the situation with chlorine, the effect of bromine does depend on the gaseous oxidant involved. This suggests that bromine compounds act to some extent by interfering with the flame reactions and it is generally believed that this is probably their principal mode of action, although they can also affect the condensed-phase decomposition of the polymer. Any gas-phase mechanism of flame-retardancy by bromine compounds must by definition involve the release of volatile brominecontaining species, which then inhibit the flame reactions. In the case of brominated flame-retardants, it is generally assumed that hydrogen bromide is liberated and reacts with the free radicals responsible for the propagation of combustion, replacing them by the relatively unreactive bromine atom. The mechanism operating in a particular polymer system will depend on the mode and ease of breakdown of the brominated flame-retardant present. Some of these compounds are thermally stable and volatilize when

26 58 the associated polymer is heated to sufficiently high temperatures. Others decompose to give substantial amounts of either lower molecular weight organic bromine compounds or hydrogen bromide [25,26]. The presence of chemically bound bromine can also affect the rates and modes of thermal decomposition of organic polymers in the condensed phase. Brominated flame-retardants vary considerably in volatility and thermal stability. both their Although some very stable compounds volatilize chemically unchanged, others break down within the polymer or react directly with it in the condensed phase. Hydrogen bromide is often a product and can significantly influence the rate and course of polymer decomposition, although its effect is small in comparison with those which it exerts on polymer combustion as a whole. However, even thermally stable brominated flame-retardants can affect the decomposition of polymers in the condensed phase, causing the original polymer breakdown stage to be replaced by two separate stages, both of which involve polymer and additive. Thus, it is clear that hydrogen bromide is not the only bromine-containing compound which affects condensed-phase polymer decomposition and that organic bromine compounds can also markedly change the rate and mode of breakdown of organic polymers [13]. A critical factor governing the effectiveness of brominated flameretardants and indeed their mechanism of action is their thermal stability relative to that of the polymers with which they are associated. The most favorable situation for a flame-retardant to be effective will be one in which its decomposition temperature lies 50 C or so below that of the polymer. In general, decomposition at this temperature with the liberation of substantial quantities of hydrogen bromide or elemental bromine is likely to enhance flame-retardant activity. Owing to the relatively low C-Br bond energy, bromine compounds generally breakdown at quite low temperatures

27 59 (typically C). Temperatures in this range overlap well with the decomposition of many common polymers. This is probably a factor determining the superior flame-retardant effectiveness of bromine compounds compared with that of chlorine compounds [26] Co-additives for use with flame-retardants Brominated flame-retardants are in some cases used on their own, but their effectiveness is increased by a variety of co-additives, so that in practice they are more often used in conjunction with other compounds or with other elements incorporated into them. Thus, for example, the addition of small quantities of organic peroxides to polystyrene greatly reduces the amount of hexabromocyclododecane needed to give flame-retardant foam; other free radical initiators behave in a similar fashion. These compounds appear to act by promoting depolymerization of the hot polymer, giving a more fluid melt. More heat is therefore required to keep the polymer alight, because there is a greater tendency for the more molten material to drip away from the neighborhood of the flame [1,27]. The flame-retardant properties of bromine compounds, like those of chlorine compounds, will be considerably enhanced when they are used in conjunction with other hetero-elements, notably phosphorus, antimony and certain other metals. The simultaneous presence of phosphorus in bromine-containing polymer systems usually serves to improve their degree of flame-retardance, with bromine and phosphorus exerting effects that are largely additive rather than synergistic. Sometimes the two elements are present in the same molecule, e.g., tris(2,3,-dibromopropyl)phosphate. In other systems, however, it is more convenient to use mixtures of a bromine compound and a phosphorus compound so that the ratio of the two elements can be readily adjusted. It

28 60 has already been pointed out that brominated flame-retardants on their own act predominantly in the gas phase. In contrast, phosphorus compounds act mainly in the condensed phase, especially with oxygen-containing polymers. It is therefore of interest to discover whether, when both elements are present together, each continues to act in the usual way or new mechanisms come into operation. However, the evidence here is somewhat conflicting. Studies of the effects of phosphate esters, with or without bromine present, on the combustion of polyesters show that more char is formed when the flameretardant contains bromine, and that most of this bromine remains in the char. This suggests that the bromine-phosphorus compound affects primarily the condensed-phase processes. However, studies of the flammability of rigid polyurethane foams show that the inhibiting effect of tris(2,3dibromopropyl)- phosphate on combustion depends on the nature of the gaseous oxidant, suggesting that the flame-retardant acts here, at least in part, by interfering with reactions in the gas phase. With hydrocarbon polymers, such as polyolefins and polystyrene, the major part of the phosphorus present volatilizes and acts in the gas phase, being apparently converted to simple species, such as phosphorus and phosphorus oxide free radicals. These species can then interfere chemically with the reactions responsible for flame propagation by catalyzing the recombination of the active free radicals involved. In such cases, any bromine present simultaneously is presumably converted to species such as Br.e and HBr which function in the gas phase in the usual way [13]. Antimony is a much more effective co-additive than phosphorus, generally in the form of its oxide, Sb2O3. On its own, this compound has no flame-retardant activity and is therefore almost always used in conjunction with a halogen compound. In general, bromine-antimony mixtures are more effective than the corresponding chlorine-antimony systems. The use of

29 61 antimony trioxide greatly reduces the high levels normally needed for effective flame-retardance of bromine compounds on their own. The principal mode of action is in the gas phase. If bromine and antimony are present simultaneously in a burning organic polymer, the major part of the antimony is volatilized, probably as SbBr3 or SbOBr. These compounds then provide a ready source of hydrogen bromide and they also produce in the middle of the combustion zone a mist of fine particles of solid SbO, which can catalyze the recombination of the free radicals responsible for flame propagation, via the formation of transient species like SbOH. A number of other metal oxides have been investigated as partial or total replacements for antimony trioxide. Their use, however, has a number of disadvantages. The most important point is that volatilization of the bromine occurs at the right stage of the combustion cycle. With zinc oxide, volatilization takes place too early and the bromine has disappeared from the system before it can become effective [28]. It can be concluded that in many, if not most, polymer systems in which bromine and phosphorus are both present, the two elements tend to act independently and therefore additively. The important mode of action of metal oxides as co-additives for brominated flame-retardants is to catalyze the breakdown of the bromine compound and therefore the release of volatile bromine compounds into the gas phase. However, metal-bromine compounds may also be formed, and these may have more specific modes of action in inhibiting polymer combustion [29,30] Smoke suppressants Smoke production is determined by numerous parameters. No comprehensive theory yet exists to describe the formation and constitution of smoke.

30 62 Smoke suppressants rarely act by influencing just one of the parameters determining smoke generation. Ferrocene, for example, is effective in suppressing smoke by oxidizing soot in the gas phase as well as by pronounced charring of the substrate in the condensed phase. Intumescent systems also contribute to smoke suppression through creation of a protective char. It is extremely difficult to divide these multifunctional effects into primary and subsidiary actions since they are so closely interwoven. At present, no uniform theory on the mode of action of smoke suppressants has been established [1] Condensed phase Smoke suppressants can act physically or chemically in the condensed phase. Additives can act physically in a similar fashion to flameretardants, i.e., by coating (glassy coatings, intumescent foams) or dilution (addition of inert fillers), thus limiting the formation of pyrolysis products and hence of smoke. Chalk (CaCO 3), frequently used as filler, acts in some cases not only physically as a dilutent but also chemically (in PVC, for example) by absorbing hydrogen chloride or by effecting cross-linking so that the smoke density is reduced in various ways. The processes contributing to smoke suppression can be extremely complex. Smoke can be suppressed by the formation of a charred layer on the surface of the substrate, e.g., by the use of organic phosphates in unsaturated polyester resins. In halogen-containing polymers, such as PVC, iron compounds, e.g., iron (III) chloride, cause charring by the formation of strong Lewis acids. Certain compounds such as ferrocene cause condensed-phase oxidation reactions that are visible as a glow. There is pronounced evolution

31 63 of carbon monoxide (CO) and carbon dioxide (CO2), so that less aromatic precursors are given off in the gas phase. Compounds such as MoO3 can reduce the formation of benzene during the thermal degradation of PVC, probably via chemisorption reactions in the condensed phase. Relatively stable benzene-moo 3 complexes that suppress smoke development are formed [1] Gas phase Smoke suppressants can also act chemically and physically in the gas phase. The physical effect takes place mainly by shielding the substrate with heavy gases against thermal attack. They also dilute the smoke gases and reduce smoke density. In principle, two ways of chemically in the gas phase exist: suppressing smoke the elimination of either the soot precursors or the soot itself. Removal of soot precursors occurs by oxidation of the aromatic species with the help of transition metal complexes. Soot can also be destroyed oxidatively by high-energy OH radicals formed by the catalytic action of metal oxides or hydroxides. Smoke suppression can also be achieved by eliminating the ionized nuclei necessary for forming soot with the aid of metal oxides. Finally soot particles can be made to flocculate by certain transition metal oxides [1] Performance criteria for and choice of flame-retardants At present, the selection of a suitable flame-retardant depends on a variety of factors that severely limit the number of acceptable materials. Many countries require extensive information on human and environmental health effects for new substances before they are allowed to be put on the market. For existing chemicals, such data are not always

32 64 available but several national and international programs are in the process of gathering this information. For most chemicals, including flame-retardants, the following information regarding human and environmental health is essential to understanding a chemical's potential hazards: 1. Data from adequate acute and repeated dose toxicity studies is needed to understand potential health effects. 2. Data on biodegradability and bioaccumulation potential is needed as a first step in understanding a chemical's environmental behavior and effects. 3. Information on the chemical's possible breakdown and/or combustion products may also be needed. 4. Since flame-retardants are often processed into polymers at elevated temperatures, consideration of the stability of the material at the temperature inherent to the polymer processing is needed, as well as on whether or not the material volatilizes at that temperature or during use. 5. Consideration should be given to the need for information on the possible formation of toxic and/or persistent breakdown products during accidental fires or incineration. Successfully achieving the desired improvement in flame-retardancy is a necessary precursor to other performance considerations. The basic flammability characteristics of the polymer to be used play a major role in the flame-retardant selection process, as readily than others. some polymers burn much more